That was it. Apparently there are three unicode controls on a NetApp filer, only two of which can be changed from the web interface on an existing volume. And you have to reboot the filer after changing the language!
The C.utf8 language worked to keep both our CIFS and NFS clients happily in Unicode filenames. Thanks.
Data ONTAP 7.3 Storage Management Guide (210-04122_A0)
Protocols in use
NFS Classic (v2 or v3) only
Language setting does not matter
NFS Classic (v2 or v3) and CIFS
Language of the clients
NFS v4, with or without CIFS
cl_lang.UTF-8, where cl_lang is the language of the clients.
Note: If you use NFS v4, all NFS Classic clients must be configured to present file names using UTF-8.